Nasa might not be able to follow the progress of its big Mars rover all the way to the surface when it attempts to land on the planet on 6 August (GMT).
The Curiosity vehicle is aiming for a deep depression known as Gale Crater.
The US space agency will be tracking the descent with satellites, but its prime craft for the task may not now be in the correct place in the sky.
Engineers have been tackling a fault on the Odyssey satellite and it is no longer in the best observational orbit.
Unless it can be moved back in the next three weeks, Nasa will lose signal to the rover just as it is about to touch down.
This will not affect the outcome of the landing because Curiosity’s descent manoeuvres are all performed autonomously, but it will give rise to some high anxiety as everyone awaits confirmation that the $2.5bn mission is safely on the surface.
“Odyssey right now looks like it may not be in the same spot that we’d expected it to be,” said Doug McCuistion, the director of Nasa’s Mars exploration programme.
“There may be some changes in real-time communication. We’ll let you know as this develops; we still have more work to do. But keep in mind, there is no risk to [Curiosity] landing. It does not have an effect on that.”
The 900kg robot’s entry, descent and landing (EDL) will be the most dangerous aspect of the entire mission.
The rover, in its protective capsule, will hit the top of the Martian atmosphere at 20,000km/h (13,000mph) and attempt to slow to just one metre per second to execute a soft touchdown.
This rapid deceleration must be achieved in about seven minutes or Curiosity will smash into the ground.
Engineers have built a complex EDL system that includes a supersonic parachute and a rocket-powered crane. Everything must work on cue and in sequence.
It was expected that the Odyssey orbiter would track the whole descent, relaying UHF signals from the rover right up to the landing and for a few minutes beyond.
But the spacecraft recently experienced a reaction wheel failure.
This device is used to manage the satellite’s orientatation and momentum in space, and because engineers have been investigating the issue they have not as yet moved Odyssey back into the correct orbit to see the full landing sequence – and they may not do so.
This would leave Nasa blind for the final, nail-biting two minutes of the landing operation. …